Magic Eye

To "solve" an autostereogram (Magic Eye) clue the viewer must shift their focus to bring out the 3D image "hidden" in the pattern. The example below shows a shark:

Stereogram Tut Random Dot Shark.png
By Fred Hsu assumed (based on copyright claims). Own work assumed (based on copyright claims)., CC BY-SA 3.0, Wikipedia

A really easy way to create one is to use an online tool such as which allows you to type your clue in its own generator and then download the image. You can can then show the image to the treasure hunters on a tablet or print it out.

For more information about using autostereograms as a treasure hunt clue see our Magic Eye post.